Conditioning the air inside a structure utilizing a gravel heat exchanger underneath the building""s foundation slab.
Conditioning the air inside of a building structure generally involves lowering the temperature of air which is being recycled in the building, or of air which is drawn in from the outside. Conventional air conditioners utilize either evaporative or mechanical refrigeration for this purpose.
The well-known xe2x80x9cswamp coolerxe2x80x9d is the most frequently encountered example of an evaporative type. Air to be cooled is passed through a mat of fibrous material over which a stream of water is pumped. The cooling effect is accompanied by a rise in humidity, a trade-off that is acceptable when cost of the system and its operations are a serious concern. It will not perform well in hot, humid regions such as in the Southern United States.
Mechanical refrigeration is usually preferred when it is affordable. The system itself is expensive, and energy to operate it is a significant continuing cost. In the operation of this type of system, the apparatus provides a substantial thermal gradient, so the volume through-put of air is often reduced, because reduction of temperature in a large room can be achieved with the introduction of a relatively small volume of very cold air.
This invention proposes the employment of a gravel heat exchanger placed beneath the foundation slab of the building. The gravel is in thermal (not necessarily direct) contact with the earth beneath it. The earth acts as a cold sink as will be explained below. Air to be conditioned is passed through the bed of gravel on its way to the interior of the building, and is cooled by heat-exchange contact with the gravel, passing through the interstitial spaces between the granules.
It is insufficiently recognized that except in very cold or excessively hot dry climes, the temperature of the earth even only a few inches deep under a shaded area is remarkably and reliably cool, and is capable of substantially maintaining its cool temperature even as it cools down the air above it, Accordingly, the earth beneath the gravel heat exchanger may be considered to be a xe2x80x9ccold sinkxe2x80x9d.
The temperature of this cold sink is usually 50-60 degrees F., which is only a modest but significant number of degrees below the usual temperatures to be reduced usually between 85 degrees F. and 120 degrees F. One immediately notices that the temperature of the cooled air often flowing through the gravel cold sink is only about 8 degrees F. higher than the 50 degrees to 60 degrees temperature of the cold sink. This is a modest gradient compared to the large gradient between the coldest temperature of the mechanical refrigerator and the same gases. It is also evident that, while a mechanical system can reduce the temperature to very low values, this invention cannot reduce it below the temperature of the cold sink.
Still upon reflection, humans are most comfortable in air above 60 degrees F., often about 78 degrees F. with modest humidity. Thus, a system whose lowest temperature reached is on the order of 70 degrees F., can still readily provide for cooling of air to temperatures in the range between about 60 degrees F. and 80 degrees F., if the air flow is increased accordingly. This is a very livable temperature range, and includes the optimum temperatures.
Basically, in this invention, this means moving air through the heat exchanger in quantities sufficient to cool the total air in the structure to the desired lower temperature. By way of example, it is estimated that the gravel heat exchanger of this invention can provide on a continuing basis about 20 watts of cooling power per square foot of earth surface. With a throughput of sufficient air, the air in the structure can in a suitably short time approach the temperature of the heat sink. Generally it will settle at about 7 degrees to 9 degrees F. above that temperature, resulting in a very livable room.
Accordingly, with the use of this invention, a greater volume of air must be moved through the heat exchanger to attain the desired lowered temperature. Here is where the only cost of operation arisesxe2x80x94the blower must force more air through the system than if the heat exchanger provided a significantly lower temperature gradient. The product of the air volume times the temperature rise must be the same for equal performance of the system. Fortunately much less energy is needed to force a stream of air through a system than to chill a body of gas in a mechanical refrigerator with the use of a gas compressor.
There is no energy expended in this process to cool the air. The only energy required is that of a blower to force the air through the heat exchanger, and this is minor compared to refrigeration costs.
The cost of the system is very affordable, requiring only clean gravel, interstitial channels in the gravel, and minor structural provisions which are made when the slab is prepared and the concrete is poured.
It is best practice to provide fine filtration of air which enters the building to assure a pure air supply. In addition, this system will cause the pressure in the building to be positive relative to the outside ambient pressure. With such a relationship, adverse material will not flow into the structure from the outside because the tendency at all structural leaks and openings is to flow outwardly. In dusty, microbial and explosive environments, this is an important safety feature. One inch of water pressure is usually adequate and readily reached.
Accordingly it is an object of this invention to provide a conditioning system which is readily and economically built, which has an extended life without maintenance other than to change or wash a filter from time to time, and which can be economically operated.
A conditioning system according to this invention is intended to be disposed below grade, preferably beneath a poured concrete foundation slab. While it can at least theoretically be installed beneath an existing slab, the work involved would be quite expensive. This invention is principally intended to be part of the initial construction, when its components can be constructed without impediment from existing structure.
It is installed in thermal contact with earth beneath it. This does not require direct contact with the earth, although it may. The preferred embodiment utilizes a thermally conductive and gas-impermeable blanket that is placed on the surface of the earth to keep the dirt, microbes and odor from entering the system. The surface of the earth provides a heat transfer surface whose temperature is responsive to that of the earth beneath it.
A gravel heat exchanger is laid on the heat transfer surface (or on the blanket atop it). This heat exchanger is a body of gravel having dimensions of depth, width and length. Entry and exit plenums are formed adjacent to edges of this heat exchanger, so that air to be conditioned is forced into and through the width of the exchanger and from it, into an exit plenum to a register or air box (including a filter) discharging into the structure.
Preferably the heat exchanger and the plenums are also shielded from the bottom of the slab, thereby becoming an encapsulated system.
As yet another optional feature, louvers may be provided to by-pass air from the structure, or to control its rate of flow into the structure.
The above and other features of this invention will be fully understood from the following detailed description and the accompanying drawings, in which: